Field of the Invention
The invention relates to a seal for retaining a CIC (completely-in-the-canal) hearing aid within portion of the ear canal by contact with the wall of the ear canal.
Description of Related Art
According to one example, such a hearing aid comprises a lateral module, a medial module and a flexible joint assembly connecting the medial module and the lateral module in such a manner that the lateral module and the medial module are movable relative to each other in order to follow the shape of the ear canal (“medial” designates the direction towards the tympanic membrane, and “lateral” designates the direction away from the tympanic membrane). According to one example, the medial module contains the hearing aid loudspeaker, while the lateral module includes the battery, the microphone and the audio signal processing electronics including an amplifier; alternatively, a third module may be provided which is connected to the lateral module and which includes one or more of these hearing aid components.
According to another example, the hearing aid comprises only a single module which includes all of the hearing aid components.
In general, CIC hearing aids of these types can be inserted very deeply into the ear canal, so that the loudspeaker can be located very close to the ear drum (a typical distance is 4 mm). Typically, such types of hearing aid (which is also known as “extended wear hearing aid) is designed to be worn within the ear canal for a few months without the need to replace the battery; such “extended wear” is achieved by minimizing power consumption.
Typically, the lateral module is primarily located in the cartilaginous part of the ear canal, while the medial module is primarily located in the bony part of the ear canal.
Such CIC hearing aids are provided with an acoustic seal surrounding the part of the hearing aid for retaining the hearing aid within the ear canal and for providing for an acoustic seal in order to prevent feedback by acoustically decoupling the hearing aid microphone and the hearing aid speaker. Such acoustic seal is made of a soft and resilient material in order to allow compression of the seal when the hearing aid is inserted into the ear canal. In particular, the hearing aid and the seal have to be designed such that the local pressure exerted on the ear canal wall is not too high; in particular, the local pressure should be smaller than the venous capillary return pressure (which is 12 mm Hg).
A hearing aid of this type is described, for example, in U.S. Pat. No. 7,580,537 B2, wherein the lateral module or both the lateral and the medial module are provided with a dome-shaped sealing retainer which is fabricated from an elastomeric foam, in particular a polyurethane foam; other examples mentioned therein are silicones, polyethylenes, fluoropolymers and copolymers thereof. It is also mentioned that all or a portion of the seal may comprise a hydrophobic layer or coating which may be made of silicones or fluoropolymers. A similar hearing aid is described in U.S. Pat. No. 7,664,282 B2.
Such seals made of hydrophilic polyurethane foam usually are net-shape molded. However, the surface to volume ratio of such seals is in disfavor of such a reaction molding method, since such reactions are usually rather fast and thus difficult to control in a very limited volume. Many parameters in such process, such as the ratio of A/B components of the polyurethane foam, the temperature of the components, the shear rate of mixing, environmental temperature and humidity, amount of mixture poured into the mold, surface properties (roughness, wettability) and temperature of the mold and the time from filling to closing of the mold (shut-off time), play a critical role for the quality of the foam, such as for the size and distribution of the pores, the skin thickness and the material density. Typically, several sizes of such seals are required for a hearing aid, so that these parameters have to be identified and controlled for each design. Furthermore, such reactive foaming process has limitations concerning minimal wall thickness or minimal feature size. In view of the usually relatively tight specifications concerning properties of the retainer seal, such reactive foaming manufacturing process has to be followed by various measurement steps, for example, concerning size, flexibility and acoustic attenuation, which limit throughput of the manufacturing site and increase cost.
It is known to produce parts made of thermoplastic elastomers and silicone rubbers by physical foaming, wherein a highly pressurized gas is injected into the molten or yet uncured polymer; thus, a porous structure is created by controlled expansion in a mold. Examples of such processes are a process by Trexel Inc., Wilmington, USA, labeled “MuCell”, and process by Sulzer AG, Winterthur, Switzerland, labeled “OptiFoam”. However, also these processes include limitations concerning the manufacturing of small parts with minimal wall thickness and an adverse surface to volume ratio.
U.S. Pat. No. 7,113,611 B2 relates to a hearing aid comprising an ear mold having a tip which is made from an injection moldable, bio-compatible thermo-elastomer, such as C-flex, or a castable, bio-compatible silicone polymer.
It is also known that porous silicone or silicone rubber structures can be produced by embedding sacrificial particles into an elastomeric matrix prior to curing of the elastomeric matrix and subsequently leaching/washing the particles, after curing, out of the elastomeric matrix, thereby generating a porous structure. It is known that such porous structures may be used as medical implants or as a substrate for growth of cells or living tissue.
Examples for the use of such porous silicone as a substrate for cells are described in U.S. Pat. Nos. 5,514,378, 5,998,185 and 6,900,055 B1.
Examples for the use of such porous silicone structures for manufacturing of implants to the human body are described in U.S. Pat. Nos. 4,969,906, 5,007,929, 4,859,712, U.S. Patent Application Publication 2011/0093069 A1, PCT Patent Application Publication WO 2009/061672 A1 and U.S. Patent Application EP 1 003 443 B1. According to U.S. Pat. No. 4,969,906, the sacrificial particles may be sodium chloride crystals having a diameter from 50 to 200 μm; such particles can be removed by water.
U.S. Pat. No. 6,391,233 B1 relates to a porous polymeric material having a porosity of more than 50%, wherein meltable organic particles melting at 100° C. to 300° C. are embedded in a polymeric matrix for generating pores. The pore-forming particles melt during the molding process, with the molded products being soaked with a solvent which dissolves the pore-forming organic material. It is mentioned that such process may be used for forming an ear plug.